Is foreign key a primary key?

A primary key is a column or a set of columns in a table whose values uniquely identify a row in the table. … A foreign key is a column or a set of columns in a table whose values correspond to the values of the primary key in another table.

Is foreign key always a primary key?

Yes, foreign key has to be primary key of parent table. Yes, it may not be unique and may have duplicate entries in child table, but it must be unique and does not have any duplicate entries at the parent table (as it is a primary key).

Is a foreign key Another type of primary key?

Primary key uniquely identify a record in the table. Foreign key is a field in the table that is primary key in another table. Primary Key can’t accept null values.

Can a primary key be a foreign key in the same table?

If you mean “can foreign key ‘refer’ to a primary key in the same table?”, the answer is a firm yes as some replied.

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Can a foreign key be non primary key?

Yes, a foreign key can reference a non-primary key which is unique. A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.

Can you have a foreign key without a primary key?

You need to have either a primary key or unique key in the column which will be referred by the foreign key. Without it you can’t create the foreign key.

How do you identify primary and foreign keys?

A primary key is a special key in a relational database that acts as a unique identifier for each record meaning it uniquely identifies each row/record in a table and its value should be unique for each row of the table. A foreign key, on the other hand, is a field in one table that link two tables together.

What is primary key vs foreign key?

A primary key is used to ensure data in the specific column is unique. A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It uniquely identifies a record in the relational database table.

What’s the difference between primary key and foreign key?

A primary key is a column or a set of columns in a table whose values uniquely identify a row in the table. … A foreign key is a column or a set of columns in a table whose values correspond to the values of the primary key in another table.

Can a column be both foreign key and primary key?

You can create a column having both keys (primary and foreign) but then it will be one to one mapping and add uniqueness to this column.

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Can a table have no primary key?

Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key. The primary key of one table may also help to identify records in other tables, and be part of the second table’s primary key.

Does every table need a foreign key?

Note that foreign keys are not mandatory, and a table may have no foreign keys. Conversely, every column in a table may have a foreign key constraint.

Can a foreign key be a secondary key?

A foreign key is a key that references an index on some other table. … That country column in the customer table would be a foreign key. A secondary key on the other hand is just a different column in the table that you have used to create an index (which is used to speed up queries).

Can foreign key be not unique?

From MySQL documentation: InnoDB allows a foreign key constraint to reference a non-unique key. This is an InnoDB extension to standard SQL. The handling of foreign key references to nonunique keys or keys that contain NULL values is not well defined (…)